The carbon refining ability of the CaO-SiO2-MnO slag was measured at 1773 K by employing a carbide capacity concept. Also, in-situ dissolution behaviour of SiC particle into the slag was directly observed by using a confocal scanning laser microscope (CSLM) at 1873 K. The carbide capacity was confirmed to be strongly dependent on the basicity and the stability of carbide ions. The carbide capacity also increased with increasing optical basicity of the slag, indicating that the optical basicity can be applied as a measuring index for the carbide capacity of MnO-containing slags. In the CSLM experiments, the SiC particles continuously dissolved into the slag through the wetting phenomena by molten slag and followed by the evolution of gas bubbles. The more basic slags enhanced the wetting of SiC particle by the slag. The fine bubbles were probably believed to be Mn(g) and CO(g) produced from the reaction between SiC particle and MnO in the slag. The more basic slag also seems to increase the apparent dissolution rate of SiC particles due to lower viscosity.